Fibrous filter media, particularly nonwoven fibrous filter media, are commonly used for air filtration. Interactions between particles and the fibrous structure when air passes through the fibrous filter medium can result in particulate capture via several mechanisms. Typically, fibrous filtration media are fairly thin and have a relatively high airflow resistance, and the flow resistance can increase significantly over the lifetime of the filter. The fibrous structures may be imparted with electrostatic charge to increase the filtration efficiency without affecting the airflow resistance.
Less common air filtration materials employ a channel-flow design, in which flow channels are formed from filter media. When air passes through the flow channels, particles are captured when they contact the filter media sidewalls. The channels may be formed in a honeycomb-like structure (hexagonal or other). Or sheet-like materials may be corrugated and laminated together to form a channel flow structure. In other cases, three-dimensional replicated films in a T-shaped configuration may be stacked to form a channel flow structure. These channel-flow structures are often, but not exclusively, formed from films, and the films or other structures may be electrostatically charged to impart an improved filtration efficiency. Some filter media with flow channels are described in U.S. Pat. No. 6,280,824 (Insley et al.) and U.S. Pat. No. 6,589,317 (Zhang et al.).
In other technologies, extrusion processes for making polymeric nets are known in the art. For example, recently extrusion dies and methods using a plurality of shims have been reported to be capable of producing polymeric netting having a thickness up to 750 micrometers. The polymeric strands of the netting are described as being periodically joined together at bond regions throughout the array. See Int. Pat. Appl. Pub. Nos. WO2013/028654, WO2013/032683, and WO2013/052371, each to Ausen et al. Also, an extruded article including an undulating strand with an aspect ratio of at least about 2 to 1 is disclosed in U.S. Pat. No. 4,634,485 (Welygan et al.).